The present invention relates to improving the efficiency of systems for producing single crystal silicon by the Czochralski (CZ) method. More particularly the invention relates to the hanging of charge replenishment rods in CZ furnaces.
The major process for producing single crystal silicon ingots for the electronic industry is the CZ process. In the CZ process chunks of polycrystalline silicon are loaded into a quartz crucible. The crucible is loaded into a furnace, which is sealed and evacuated. The polycrystalline silicon is melted under vacuum and once the melt has been stabilized a single crystal silicon seed of the correct orientation is inserted into the melt. A silicon single crystal ingot of the correct diameter and orientation is pulled from the melt. This ingot is used to produce silicon wafers, which are the major starting material for the electronics industry. Wafers from the CZ process are used predominately for the production of integrated circuits.
One of the limitations of the CZ process has been that the quartz crucible can be used only a few times. This is due to the differences of thermal expansion of the polycrystalline silicon and the quartz so that once the CZ process is finished and the heat is removed, it is common for the quartz crucible to crack.
In order to address this issue, a number of attempts have been made at evaluating alternative crucible materials. These have included coating quartz with Si3N4 or other ceramic materials. These have met very limited success due to the overall purity requirements of single crystal silicon.
Another approach that has been attempted is to extend the length of time a quartz crucible can be used. This approach includes development of semi-continuous CZ crystal growth processes. A semi-continuous process typically involve the use of a feeding tube through which small polycrystalline silicon chips or fluid bed (granular) polycrystalline silicon can be fed. In such a semi-continuous process the crucible can be topped off with chips or granular polycrystalline silicon. Or an ingot could be pulled, then removed, and additional polycrystalline silicon added to the remaining melt. In this manner, the crucible can be used for an extended period. Unfortunately, due to the potential for surface contamination that occurs with chips and granular polycrystalline silicon, such techniques have not won wide-spread acceptance in the industry.
Another alternative is to recharge the crucible with rods of polycrystalline silicon. This process is called charge replenishment (CR). Typically, a ring ditch is fabricated onto the cylindrical surface of a polycrystalline silicon rod and a holder is fastened to the ring ditch. The holder is attached to the seed holder of a CZ furnace so the assembly can be raised or lowered in the furnace. Thus the rod can be slowly lowered into the silicon melt and melted in. In this manner the crucible can be refilled with molten silicon, without reducing the temperature to the extent that the crucible cracks, such that another single crystal ingot can be pulled. This replenishment process can be repeated.
The current processes for recharging with rods have procedural limitations. For example, the loading of a new rod into the CZ furnace requires segregating the silicon melt from an upper chamber of the furnace by operation of an isolation valve. This requires time and has the potential for contamination from the isolation valve since it must be opened and closed several times. Each time the isolation valve is opened or closed it has the potential to drop impurities into the silicon melt. And once the polycrystalline silicon is melted the holding mechanism needs to be removed via the upper chamber. This requires isolating the melt with the isolation valve and removing the fastening equipment and the unmelted polycrystalline silicon residue. Then the seed can be attached and the upper chamber is evacuated. The isolation valve then must be opened so the seed can be dipped into the melt.
The current procedures thus may have one or more problems, such as delays in the crystal growing process due to the need for isolation from the molten silicon and the attachment of the rod and the removal of the rod holder, and the number of times the isolation valve must be opened and closed, which increases the potential for contamination of the silicon melt.
Thus there remains a need for ways for CZ crystal growers to increase their yields by using CR rods to increase the melt volume and therefore increase the length of the single crystal ingot they can produce. In addition for those that are already using CR rods, there is a need for ways to allow for the complete utilization of the CR rod and for reducing the number of times the isolation valve has to be opened.